1. Field of the Invention
The present invention relates to analysis of energy propagated in the form of waves, and in particular, to source location of such energy, especially in one or two dimensional systems which can be dispersive and which, during physical changes in structure or in use, will produce or carry energy such as acoustic signals or electromagnetic waves.
2. Problems in the Art
There has, of course, been long and continuous scientific study of the propagation of wave-like energy through conducting media. From such study, valuable practical applications have emerged.
For example, ultrasound energy has been widely used as a non-destructive evaluation tool. Imaging of internal parts of the human body are possible. Ultrasonic scanning of metal, for instance airplane wings, for cracks or fatigue without altering the metal can be done.
As much as these studies and applications to real world practice have advanced, there is still room for improvement in the understanding and uses of wave-form energy. Moreover, as an adjunct, a better understanding of such energy has opened up possibilities regarding detection and location of the source of such energy in a conducting medium.
For example, it would be valuable to know when and where physical changes in structures occur. Specifically, it would be advantageous to be able to monitor crack initiation location in flat plates , rods, and other mechanical structures, or leak location in underground storage or delivery systems, such as pipes. There have been many attempts to non-destructively accomplish such monitoring, detection and/or location. However, most methods are not consistently as precise as desired or accurate at locating a source of generation of energy carried in the form of propagating waves. Also, many current systems provide acceptable accuracy for limited situations, and then only if expert and experienced personnel interpret the results.
Consider the following situation. Above and under ground storage tanks present significant environmental hazard potential for certain stored fluids. Approximately one-half of the drinking water in the United States is derived from ground water. Even small amounts of gasoline, which includes benzene and other suspected carcinogens, leaked into the ground can contaminate millions of gallons of potable water. Vapors can also reach sewage systems and into the air. Estimates of the cost of potential needed remediation are on the order of thirty billion dollars.
The scale of this problem is immense. It has been reported that on the order of 250,000 leaks or releases were confirmed in 1997 out of the 1.8 million regulated underground storage tanks and pipelines in U.S. Based on these figures, almost 14% of those structures leaked last year. It has been estimated that 15-20% are leaking or will leak shortly. Some sources claim that as many as 750,000 above ground tanks have potential imminent problems. There are also a great number of below ground tanks. Many of the storage tanks are quite large in size. There are also thousands of miles of pipelines that carry petroleum based or other fluids (liquid or gas) that are potentially hazardous to the environment. Examples of the locations for these structures include gas stations, airport fueling stations, military fueling depots, waste management systems, nuclear power plants, and chemical manufacturing facilities, to name a few.
As a result of these risks, federal and state laws have been enacted that require periodic inspections and testing of such storage facilities in the hope that potential or actual leakage problems will be found as early as possible. Thus, especially underground tanks and pipes, are now tested for leaks on a regular basis and remedies must be implemented if any leak is found. Such remedial action can be very costly.
Current leak detection and/or leak location technology is inaccurate and inefficient. In the example of underground pipelines, the need is to provide the location of a leak to within the width of a backhoe (approximately 6 ft.), so that only one hole must be dug to allow correction of the leak. If a predicted leak location is off by more than this, the time and expense of digging multiple holes is incurred.
Discussed below are some of the current ways used to detect and/or locate leaks in pressurized storage tanks or conduits, such as pipelines.
Volumetric and Pressure Based Methods. These methods attempt to track line volume and line pressure for product loss due to leaks. The methods only work for significant leaks (1-5% losses). They are also limited to the spacing of the sensors used. Volumetric and pressure based methods are acceptable for highly pressurized lines (e.g. steam lines from boilers) to detect, but not to locate leaks. Additionally, false alarms are a problem. Also, they require expert, experienced personnel to provide a reasonable degree of accuracy. These methods are also extremely disruptive of the normal operations of the systems. For example, the U.S. Department of Energy (DOE) tests for leaks by going off-line and pressurizing the pipes with gas. Thus, an opportunity cost is incurred because of the loss of normal operation time in the pipeline. Also, cost considerations for administering such testing (including equipment costs), are important, especially for small businesses, e.g. gas stations.
Electrical Methods. Wire, tape or cable is installed along the pipeline. The methods monitor for a fluctuation in the signals picked up by the wire, tape or cable caused by a leak, and use the time delay that can be measured at receipt of the fluctuation to predict the location of the leak. While this is fairly effective, it is costly because it must be physically installed along the entire pipeline. It is probably cost-prohibitive to retrofit most existing underground pipelines. Corrosion and other environmental factors can damage or effect its performance, including the creation of falsings. It would then be costly and sometimes difficult to track down the precise location of the damaged wire.
Optical methods. Infrared (IR) spectroscopy methods are sometimes used, especially if the product being conveyed is warmer than the soil surrounding the pipeline. An advantage is that above ground sensors can be used. However, they must be first calibrated to non-leak situations. Also, these methods are unreliable. Almost all factors and conditions involved with underground pipelines can affect the accuracy or reliability of these types of methods (e.g. soil type, pipe design, temperature of product, amount leaked).
Tracer gas. Helium or sulfur hexaflouride, for example, can be introduced through the pipeline. Like optical methods, sensing devices to detect leakage of the tracer gas can be above ground. If such gas escapes through a leak in the pipe, the theory is it will then leak through soil and be detectable. However, these processes require system shut-down, and draining of the normal product that is conveyed through the pipeline. There is imprecision in ground level detections, because of diffusion of gas. The gas may not travel vertically, and therefore detection of the gas above ground will not necessarily provide accuracy regarding location of a leak. Because of this imprecision, workers may have to bore many holes to find the area of highest concentration of the leaking gas, to then find the location of the leak.
Because of the short-comings of the above type methods and the potential environmental hazards discussed above, early in the 1990's the EPA began to focus on acoustic detection methods. Leaks generally are very small in size, and therefore turbulence at the leak location by the leaking fluid creates sound energy. The pipeline, and the fluid, are usually acoustically conducting. Sensors can be placed on the pipeline and "listen" for such sound.
Two types of acoustic detection methods have evolved. One is called attenuation-based (reduction in signal amplitude with increasing distance from the source). The other is time of flight based (increase in signal transit time with increasing distance from the source).
One time of flight methodology placed acoustical sensors along the pipeline until the sensors bracketed the source. Cross-correlation, well known in the art, is then used to take a numerical measure of shifted signal correlation to determine location of the source. However, the above is based on the assumption that the received signals are merely time-shifted replicas of the original noise signal generated at the leak. If the signal degrades over time, correlation would be severely degraded, and the estimate of leak location would be materially affected. The causes of degradation could be, for example, (a) frequency dependent attenuation, (b) multiple possible modes of wave transmission, and (c) velocity dispersion with each mode (i.e. frequency dependent wave velocity).
All other systems require bracketing of the leak. While cross correlation techniques are still used,these methods alone still do not account for the multi-modal dispersion. Pipes are particularly highly dispersive, which severely affects the temporal dependence of the source signal. The greater the distance from the source, the greater the effects of dispersion on the signal. If the medium is non-dispersive, cross correlation works fairly well. The operator finds a peak in the signal after cross-correlation. The peak indicates time delay and by knowing the speed of sound through the pipe, location of the source of sound can be derived. If the medium is dispersive, however, it is harder to determine the peak or time delay from the signal received because of multiple modes of propagation that will be in the received signal.
Therefore, current acoustic emission methods, are advantageous because they are passive, non-destructive, can be used on existing pipeline, and are not hugely costly. These methods compare signals from pairs of acoustic transducers. However, it truly is an art to divine results from the signals. One needs to be expert. But even an expert will be unable to create a multi-modal analysis with a degree of accuracy. Also, background and system operational noise deflates signal-to-noise ratios (SNRs), depending on frequency. Current acoustic emission systems can not detect desired limits of leak size (0.1 gal/hr) unless sensors are very close and ambient noise fairly low. It is difficult to separate ambient noise from the signal produced by the leak. The best signal to noise ratio is when the leak is noisy compared to ambient noise or operational noise. This can not be controlled, most times. Therefore, even acoustic detection will probably be done during system shutdown to reduce ambient noise.
Thus, even though acoustic emission location detection methods are promising, there are problems or deficiencies in the art.
Objects, Features, Advantages of the Invention
It is therefore a principle object of the present invention to present a method and apparatus which solves or improves over the problems or deficiencies in the art.
Some specific objects, features or advantages of the present include an apparatus and method which:
a) can be used to monitor all media which conducts energy propagating in waves for such energy;
b) can be used to monitor leak location in underground or submerged storage;
c) can be used to monitor leaks in underground or submerged pipeline;
d) is more sensitive and more accurately estimate source location of propagating waves in a medium;
e) work using any wave propagation mode type (symmetric or non-symmetric);
f) isolate a particular mode or modes, even in the presence of a large number of modes;
g) is non-intrusive, and non-destructive;
h) allows relatively few sensors;
i) is retrofittable to existing structures;
j) do not need experts, but rather is operable by technicians;
k) can be portable or automatic;
l) does not require down time or evacuation of lines or containers;
m) can both detect and locate sources;
n) can pick out the location of propagating waves from ambient or operational noise in signal processing;
o) takes into account propagation characteristics of the medium, including highly dispersive medium;
p) is relatively economical and efficient.
These and other objects, features and advantages of the invention will become more apparent with reference to the accompanying specification and claims.